I don't use Photoshop, but I use Paint Shop Pro 9 and the keyboard shortcuts for those are:
CTRL+M - reflect in the y-axis
CTRL+I - reflect in the x-axis
Reflecting in the x axis means something pointing downwards will point upwards. Try both of these, and If it doesn't work, go to the Help section, and try typing in 'Mirror', 'Flip', 'Reflect' etc without the quotations.